The present invention relates generally to refrigeration and more particularly to a cryogenic cooling system useful for cooling, for example, a superconductive device. For purposes of describing the invention, the term "cryogenic" is defined to describe a temperature colder than generally 150 Kelvin.
Superconducting devices include, but are not limited to, magnetic resonance imaging (MRI) systems for medical diagnosis, superconductive rotors for electric generators and motors, and magnetic levitation devices for train transportation. The superconducting coil assembly of a superconducting magnet of a superconductive device includes a vacuum enclosure containing one or more superconductive coils which are wound from superconductive wire and which may be generally surrounded by a thermal shield.
Some superconductive magnets are conductively cooled by a cryocooler coldhead (such as that of a conventional Gifford-McMahon cryocooler) which is mounted to the magnet. Such mounting of the cryocooler coldhead to the magnet creates difficulties including the detrimental effects of stray magnetic fields on the coldhead motor, vibration transmission from the coldhead to the magnet, and temperature gradients along the thermal connections between the coldhead and the magnet. Such conduction cooling is not generally suitable for cooling rotating magnets such as a superconductive rotor.
Other superconductive magnets are cooled by liquid helium in direct contact with the magnet, with such liquid helium boiling off as gaseous helium during magnet cooling and with such gaseous helium typically escaping from the magnet to the atmosphere. Locating the liquid helium containment inside the vacuum enclosure of the magnet increases the size of the magnet system which is undesirable in many superconductive magnet applications.
What is needed is an improved cryogenic cooling system useful for cooling a superconductive device. Further, the cooling system must be remotely located from the magnet. Additionally, the cooling system should be capable of cooling multiple superconductive coil assemblies and should be capable of cooling a rotating superconductive magnet such as that of an electric generator rotor.